Limiter circuit for frequency modulation receivers



Dec. 11, 1945. c. E. ATKINS 2,390,503

.LIMITER CIRCUIT FOR FREQUENCY MODULATION vRECEIVERS Original Filed Jan. 7, 1942 INVENTOR Cm; E. A r/mus.

BY Bum; 9m,

ATTORNEYS v Patented Dec. 1945 Lmrrna cmcurr Foa s immer MODULATION ancslvnns Carl Edward Atkins, Evanston, Ill., assignor to Tong-Sol Lamp Works, Inc., Newark, N.-J., 'a corporation of Delaware Original application January I, 1942, Serial No.

425,843. Divided and this application Decemduration impulse n ises.

ber 31, 1942, SerialNo. 470,780

4 Claims.

limiter circuit for incorporation into radio re-,

ceivers'of this general type. In frequency mod a constant.

a predetermined value.

ing of which:

Fig. 1 is a diagrammatic circuit drawing or a limiter circuit embodying the invention and utilizing a pair of diodes as a cyclical voltage ate; and n Fig. 2 is a circuit drawing similar to Fig. 1 but including a regenerative link for neutralizing This grid 4 is also connected with Grid 6 of tube 2 is connected Anode Thus 5 ulated radio reception, as distinguished from amthe small amount of degeneration occurring when plitude modulated radio reception, the receivers I the diodes are conducting. should respond only to the rate or change of an- The limiter circuit of Fig. 1 includes the multigle oi the incoming wave, that is, to the relative element radio frequency amplifier tube 2 havins frequency variation of the wave and should not two grids-4 and 6 in addition to the control grid be responsive to excessive amplitude variations 8 and usual screen grid ID. A suitable input ciroi' the wave. Accordingly, receivers of this type cult for the tube 2 is indicated as the conveninclude in one form or another a device for suptional tuned circuit having the inductance L pressing amplitude variations above a predeterbridged by the condenser C, the circuit being mined value, which excess variations are caused is grounded at one end and connected to the conby static or noise impulses. Such devices are trol grid 8 at the other end. A resistor ii,

A known as lim ters" and consist principally of one grounded at one end and connected at its other i or more radio frequency amplifier stages operend to the cathode ll of the tube 2 and bridged ated under conditions of poor eilic ency so that by condenser C1 serves as a bias for the control the gain per stage is necessarily low and the stage grid. 'Grid 4 is connected to the plateili of tube will overload when asignal 01' any magnitude is 2 through a radio frequency impedance l8, speapplled to it. Usually these stages include re ciflcally shown as an inductance, and a blocking sister-condenser networks in the amplifier tube condenser C2.

' grid return. Various disadvantages are inherent an anode 20 of one of a pair of diodes forming in the above ty e of l miter. It is not able to folthe cyclical voltage gate of the feed-back. The low rapid impulses because of t e time consumed diodes are shown as elements of a tube 22, such in dischar e of the condenser through the resistas a 6H6. A lead 24, containing a radio fre- -ances. and he ce is not effective to'suopress short. quency choke 26, is connected at one'end to a Also. there is a-tendcenter tap of the resistance l2 and at its other ency, after becoming severely overloaded. for the end to the grid 4 and anode 20 through the inlimiter output to diminish instead of remaining ductance It. The cathode 28, forming the diode with anode 20, is grounded and hence a pos- An obiect of t e present invention is thus to itive potential corresponding tothe potential drop overcome these diiiiculties by providing a limiter across half of the resistance I2 is impressed circuit that will completely su press all ex essive across the diode. am litude va iations. and which will be inherdirectly to the cathode 30 of the other diode and ently rapid in res onse and hence will'suopress ,through a rad o frequency choke 32 and the noise impul es of short. as well as long. duration. condenser C: with the plate It of tube 2.

In the limiter of the present invention. the re- 34 of this diode is connected to the cathode I 4 sistance-condenser network is omitted and alfeed-' 4o of tube 2 and hencea positive potential correback is provided between output and input utisponding to the potential drop across the other lizing non-l near imoedanccs as a cyclical volthalf of resistance l2is impressed across this d1! are ate for insuring degeneration oi. the amode. pliiier tube when the amplitude variations exceed The radio frequency impedances I 8 and 32 have magnitudes high in comparison to the im- According tothe invention. the non-linearimpedance of the diodes during their respective pcdances are used to pass current to thwart deconducting cycles but low in comparison with the generation of the in ut voltage until the output respective grid impedances of the tube 2. volta e exceeds a certain value after which time the circuit including impedance l8 and the diode no current is fed back through the impedance comprising the anode 20 and cathode 28 is a and degeneration of the input voltage results. voltage divider for potential fed to the grid 4 oi For a'better understanding of the invention, tube 2 while the impedance 82 and the other direierence may be had to the accompanying drawode is likewise a voltage divider for potential fed I to the grid 8 of tube 2. It the resistance I2 is designed for a potential of say 3 volts, then with the connections above described the potential across each diode will be one and one-half volts and the full three volts across resistance l2 serves as a bias for the signal grid 8 connected to the tank circuit LC.

During operation of the above described circuit, when the input voltage does not exceed the predetermined value, the diodes or the tube 22 are conducting and hence degenerative potentials are not applied to the grids 4 and 6 of tube '2. During any cycle, however, if the peak voltage is such that the voltage across the impedance Z of the output circuit overrides the biasing voltage across either diode, then that diode becomes nonconducting and substantially the full output voltage is applied as a degenerative potential to the corresponding grid. Thus the diodes operate as a cyclical voltage gate to instantly limit the voltage gain of the amplifier 2 to the value determined by the constants of the circuit.

4 In the above described circuit of Fig. 1 there will necessarily be a small amount of degeneration present even when the diodes are conducting as the impedances of the diodes, when conducting, are not entirely zero. In order to neutralize such degeneration, a small amount of in phase voltage may be fed back to the input circuit, and such an arrangement is illustrated in Fig. 2.

The circuit of Fig. 2 differs from that of Fig. 1 only in the addition of a regenerative link 3% including the inductances La and L4 inductively coupled respectively with the inductances L5 and I L in the output and input circuits; the output impedance being shown as including the tuned circuit L5, Cs.

The operation of the circuit of Fig. 2 is the same as that of Fig. 1 except that the regenerative feed-back neutralizes the small degenerative effect occurring when the diodes pass current.

From the above description of the circuit diagrams of Figs. 1 and 2 it will be apparent that the invention provides an efiicient and rapidly operating limiter circuit for receivers of frequency modulated radio waves. Degeneration of the radio frequency amplifier inputis insured whenever the output voltage exceeds the threshold value and this degeneration is controlled by a cyclical voltage gate in the feed-back circuit which opens at output voltages below the threshapplication Serial No. 425,843, filed January 7, 1942.

I claim:

1. A limiter circuit for radio receivers comprising an'amplifler having a cathode, a control grid and at least two other grids, an input circuit connected to said control grid, an output circuit having an impedance-therein, said other two of diodes, the anode of one oi which is connected old value to pass current for thwarting degeneration. Thev particular cyclical voltage gate or non-linear impedance illustrated in the drawing is a pair of diodes but obviously other devices, such as crystal detectors or rectifiers, could be substituted therefor.

This application is a division of my copending to one of said grids of the amplifier and the cathode of the other of which is connected to the other of said grids and wherein said means for applying potential diflerences to said impedances comprisesa voltage divider connected at one end to the cathode of the amplifier, and to the anode of the last mentioned diode, grounded at its other end and connected at its midpoint through a radio frequency choke and through the radio frequency impedances with said grids of the amplifier, the cathode of said first mentioned diode be:

ing grounded;

3. The limiter circuit according to claim 1 wherein the output and input circuits each includes an inductance and wherein a regenerative feed back circuit couples said inductances togetherfor. suppressing degeneration due to the impedance of said non-linear impedances during conduction.

4. A limiter circuit for radio receivers comprising in combination, an amplifier, input and output circuits connected therewith, reed-back means connecting said circuits for impressing degenerative potentials on the input'circuit when the output voltage exceeds a predetermined value, said feed-back means including a pair of diodes biased to become non-conductive when the output voltage exceeds the predetermined value, and a regenerative link between said output and input circuits to compensate for the impedance of said diodes when conducting.

CARL EDWARD ATKINS. 

